i893. ARACHNID AND INSECT DEVELOPMENT. 443 



system of the adult King-Crab is remarkably distinct in appearance 

 from that of any other arthropod, and " shows a profound structural 

 similarity to that of the vertebrates." In order to see this, one must 

 " tear off the deceptive arthropod mask that disguises Limulus." The 

 true value of Dr. Patten's homologies can only be estimated by 

 students of the vertebrate brain. But more evidence will be required 

 to convince most naturalists that the many profound structural dis- 

 tinctions between arthropods and vertebrates do not point to a wide 

 divergence of origin. In all arthropods the oesophagus pierces the 

 nervous system. Dr. Patten suggests that in the primitive arthropod 

 ancestor of vertebrates the oesophagus " broke through the narrow 

 band of nerve-tissue in front of it." But, unfortunately, in Liinulus, 

 which he regards as the nearest living representative of this ancestor, 

 the nerve-tissue in front of the oesophagus is not a narrow band, but 

 forms the great cephalic lobes ; and so, if this explanation is to be 

 accepted, the correspondences between these lobes in Limulus and the 

 vertebrates must be homoplastic, and not homologous. 



Dr. Patten gives interesting details of the structure of the sense- 

 organs of Limulus. Sensory spines on the joints of the legs are 

 covered with tubular openings, each containing a chitinous tubule in 

 connection with a single elongate nerve-cell. These have a tasting 

 function. In the organ of 'smell are found clusters of large cells 

 surrounding a multipolar ganglion-cell, connected with a chitinous 

 tube ending near the outer surface. 



At the end of his paper, Mr. Kingsley reviews the question of the 

 systematic position of Limukis. In a tabulated statement, he 

 instances but six points in which its development and adult structure 

 agree with those of the crustaceans, while the points of agreement 

 with the arachnids number twenty-eight. He joins Liinulus with 

 the Eurypterids as a sub-class Gigantostraca forming, with the sub-class 

 Arachnida, the class Acerata. With such strong confirmation of its 

 arachnid affinities, most naturalists will prefer to call Limulus simply 

 an arachnid. The association of the Eurypterids with the same class 

 is justified by the researches recently summarised for us by Mr. 

 Laurie.^ But the Trilobites, for some time past considered as related 

 to these forms, must probably be restored to the Crustacea since the 

 discovery of their antennae. 5 



Mr. Kingsley proceeds to the discussion of the classification of 

 the Arthropoda generally. The Acerata and Crustacea form a sub- 

 phylum Branchiata. The sub-phylum Insecta includes classes 

 Hexapoda and Chilopoda while the Diplopoda form a separate sub- 

 phylum. This view, that the centipedes are more nearly related to 

 the true insects than to the millipedes, and that the Myriapoda must 

 therefore be considered an unnatural group, has been put forward 

 before, but has lately been elaborated also by Mr. Pocock (5) who 



•* Nat. Sci., vol. iii., p. 124. ^ Nat. Sci., vol. iii., p. 162. 



